1. Field of the Invention
This invention relates to the production of molybdenum oxide (MoOx). In one aspect the invention relates to a multi-step, hydrometallurgical, pressure oxidation process for the conversion of molybdenite (MoS2) to MoOx, preferably molybdenum trioxide (MoO3), while in another aspect, the invention relates to such a process that includes steps of solvent extraction, aqueous ammonia recycle, rhenium recovery and selenium removal.
2. Description of the Related Art
Molybdenum is a grey metal or black powder with many uses. One use is as an alloying agent in the production of stainless and low alloy steels to enhance toughness, high-temperature strength and corrosion resistance. Another use is as a component in oil refining catalysts used in the removal of sulfur from fuel.
Molybdenum does not occur free in nature but typically as a sulfide. Molybdenum disulfide (MoS2), also known as molybdenite, is commonly found in association with copper ores, and it is separated and concentrated from other minerals in the copper ore by froth flotation.
Typically, at first, a bulk copper/molybdenum concentrate is produced. The molybdenite concentrate is then produced by suppression of other minerals like chalcopyrite and pyrite. Molybdenum disulfide is then up-graded in a number of sequential cleaning steps. The molybdenite concentrate is filtered and dried for further processing.
Molybdenum is typically sold as an oxide which requires converting the molybdenite to a molybdenum oxide. The most common process for converting molybdenite to molybdenum oxide is roasting in which the concentrate is heated while in contact with an excess air to form molybdenum trioxide and sulfur dioxide as a gaseous by-product.
Another process for converting molybdenite to molybdenum oxide is the hydrometallurgical process of pressure oxidation. U.S. Pat. No. 6,149,883 teaches the production of molybdenum trioxide from molybdenite by a pressure oxidation process comprising the steps of forming an aqueous slurry of molybdenite, pressure oxidizing the slurry to form soluble and insoluble molybdenum species, converting the insoluble molybdenum species to soluble molybdenum species by alkaline digestion, separating the soluble molybdenum species from insoluble residue contaminants (if present), removing the molybdenum species from the aqueous media through solvent extraction, recovering the molybdenum values from the organic solvent as soluble ammonium dimolybdate by stripping with aqueous ammonia, recovering ammonium dimolybdate crystals by evaporative crystallization, and recovery of molybdenum trioxide (MoO3) by thermal decomposition (calcination). Low grade molybdenite concentrates, including concentrator slimes containing talc and sericite, can be used as a feed. The process produces both technical and chemical grade molybdenum trioxide.
The process of the '883 patent, while effective, has room for operational and capital cost improvement. For example, improvements in the recovery of ammonia, the recovery of rhenium, and the reduction in the discharge of selenium all are of interest to the producers of molybdenum oxide. In addition, any reduction in energy usage and/or environmental footprint is a welcomed addition to the overall efficiency of the process.